Android存儲之一利用外部存儲卡存儲數據

佈局文件：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/tv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

寫入數據到sd卡中：

1. 創建一個File文件路徑；
2. 創建一個File文件；
3. 向文件中寫入數據；

  File dir = Environment.getExternalStorageDirectory();
  File file = new File(dir, "data.txt");

 FileOutputStream fos = new FileOutputStream(file);
                fos.write("hello Geoff".getBytes("utf-8"));
                fos.flush();
                fos.close();

從文件中讀取數據；

 FileInputStream inputStream =new FileInputStream(file);
            byte[]bytes =new byte[inputStream.available()];
            inputStream.read(bytes);

給控件賦值

tv.setText(new String(bytes));

代碼如下：

package com.example.sdsave;

import android.os.Bundle;
import android.os.Environment;
import android.widget.TextView;

import androidx.appcompat.app.AppCompatActivity;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

public class MainActivity extends AppCompatActivity {
    private TextView tv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv=findViewById(R.id.tv);
        File dir = Environment.getExternalStorageDirectory();
        File file = new File(dir, "data.txt");
        if (!file.exists()) {
            try {
                file.createNewFile();
            } catch (IOException e) {
                e.printStackTrace();
            }

        } else {
            try {
                FileOutputStream fos = new FileOutputStream(file);
                fos.write("hello Geoff".getBytes("utf-8"));
                fos.flush();
                fos.close();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }

        }
        try {
            FileInputStream inputStream =new FileInputStream(file);
            byte[]bytes =new byte[inputStream.available()];
            inputStream.read(bytes);
            tv.setText(new String(bytes));


        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }


    }
}

最後記得在AndroidManifest.xml加入權限：

   <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission>
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"></uses-permission>

這是向內存卡中寫入權限和讀取內存卡的權限；